1. Field of Technology
The present invention relates to an image forming method, optical scanning apparatus and image forming apparatus wherein a laser beam is applied onto a photoreceptor to perform scanning and exposure operations according to image data by means of an optical scanning apparatus, whereby an image is formed.
2. Description of Related Art
In an image forming apparatus of a printing press, photocopier, printer, facsimile, and MFP (multi-functional peripheral) made up of a combination of these devices, a pulse width modulated signal having a modulated pulse width is generated according to the image data containing density information. This signal is inputted into the laser beam generating section (e.g., LD) as a drive signal. The laser beam coming out of the laser beam generating section is applied onto a photoreceptor to perform scanning and exposure operations by an optical scanning apparatus so that a latent image is formed on the photoreceptor. After that, the latent image is developed by a developer and is fixed onto a sheet of paper, whereby the image forming operation is performed.
Generally, the laser beam having passed through a cylindrical lens is deflected in the main scanning direction by a rotating polygon mirror, whereby the aforementioned optical scanning apparatus performs a scanning operation. An fθ lens is arranged between the polygon mirror and photoreceptor, and the deflected laser beam is converged onto a flat image surface, whereby a scanning operation is performed at a uniform speed.
Incidentally, in the aforementioned image forming apparatus, it is difficult to ensure the uniform position in the main scanning direction (image height) and uniform diameter of the laser beam formed on the photoreceptor in the main scanning direction, partly because the laser beam scanning operation is performed in the main scanning direction. Thus, the fluctuation thereof and the fluctuation in the output power of the laser beam are known to cause irregularity in the density of the output image.
One of the methods proposed to solve this problem is a feedback control technique for detecting the output power of the laser beam and feeding the same back to the output at the laser output section (Japanese Unexamined Patent Application Publication No. 2002-67376). This technique, however, fails to prevent the irregularity in density from being caused by the fluctuation in the diameter of the laser beam in the main scanning direction.
In an image forming apparatus having been proposed, a sensor for detecting the beam diameter is mounted on the photoreceptor, and an appropriate pulse width is selected so as to ensure exposure energies in the main scanning direction will be the same in response to a plurality of different positions in the main scanning direction (a plurality of different image heights) and the beam diameter in the main scanning direction, whereby the irregularity in density is prevented. In the selection of the appropriate pulse width, a table is stored to show the appropriate pulse width to be used in response to the number of beam diameters, if a plurality of beam diameters is present in the main scanning direction (see the description of the Embodiments and others in Japanese Unexamined Patent Application Publication No. 2000-190554).
In a study made by the present inventors, however, it has been found out that, in addition to the fluctuation in the diameter of the laser beam in the main scanning direction resulting from the laser scanning operations in the main scanning direction, the fluctuations in the diameter of the laser beam in the sub-scanning direction is caused by the geometric precision of the cylindrical lens and fθ lens from the laser beam output section to the photoreceptor. This is known to affect the irregularity in the density of the output image. It has also been found out that the latter fluctuation tends to be greater than the former fluctuation. This finding is the result of the aforementioned study made by the present inventors wherein, in the laser beam scanning operation, the fluctuation in the diameter of the laser beam in the main scanning direction was observed separately from the fluctuation in the diameter of the laser beam in the sub-scanning direction. Especially when a plastic lens is used as the fθ lens, the fluctuation in the sub-scanning direction is sometimes several times greater than that in the main scanning direction, for example. The magnitude of the beam diameter in the sub-scanning direction as described above has a more serious impact on the irregularity in density. The proposed aforementioned techniques have failed to sufficiently avoid the irregularity in density.
The present invention has been made to solve the prior art problems described above. To ensure uniform output image density, the primary step to be taken is to consider the problem from the viewpoint of fluctuations in the sub-scanning direction. This attitude has led to the completion of the present invention.